WhatsApp has 2 billion DAU; 50% send messages on a given day, 10 messages each. For connections, assume each user's app holds an open connection for 5 minutes (300 s) per day, spread uniformly across the day.
Use: 1 day = 86,400 sec ≈ 10^5 sec. Powers of 10: 1 KB = 10^3 B, 1 MB = 10^6 B, 1 GB = 10^9 B, 1 TB = 10^12 B, 1 PB = 10^15 B.
Compute message volume, write QPS, peak QPS, and average concurrent connections.